1. Field of the Invention
The present invention generally relates marine engines and, more particularly, relates to marine engines used in motors designed for low speed trolling operation.
2. Description of the Related Art
Outboard motors frequently propel watercraft while running at an engine speed slightly above or slightly below a neutral idle engine speed. Such operation is commonly called trolling. During trolling, a conventional engine control unit (ECU) for the outboard motor seeks to achieve a target engine idling speed. The ECU may manipulate a secondary air valve that opens and closes an air bypass around the main throttle valve such that the idling engine speed, or trolling engine speed, can be adjusted higher or lower.
In some instances, the target engine speed is determined based upon a reference engine speed stored in memory and is able to be adjusted based upon operator input. In other words, a reference engine speed is used unless that reference engine speed is increased or decreased by manual input from an operator of the outboard motor. In many instances, the reference engine speed is determined based upon a detected engine operating temperature with the reference engine speed generally decreasing as the engine operating temperature increases.
Outboard motors are typically water-cooled. Since watercraft are designed to float upon bodies of water, the surrounding water is a convenient source of cooling water for outboard motors. Thus, open loop cooling systems are common within the industry. The open loop cooling systems, however, sometimes deliver water that is substantially colder than the engine was designed and the colder water can retard the warming up of the engine. In such arrangements, the assumed engine temperature may be higher than the actual engine temperature. Thus, the ECU may be fooled into believed a warmed-up condition has been achieved and may set the idle speed lower than desired for the actual engine operating temperature. The lower idle speed can cause the engine to stall due to the relatively higher than expected friction forces in the engine due to the lower temperature.